1. Field of the Invention
This invention relates to a pulse oximeter for continuously measuring a degree of oxygen saturation of arterial blood of living tissue by light emitting and receiving elements in non-invasive condition, more specifically, relates to a drive circuit, used for the pulse oximeter, for driving two light beams emitting elements to transmit two light having different in wavelength to the living tissue.
2. Prior Art
In the conventional art, a pulse oximeter is used for measuring an oxygen saturation of arterial blood in non-invasive condition. This type of the pulse oximeter operates to measure the oxygen saturation as follows. Red and infrared light different in wavelength have been irradiated to an object and the oxygen saturation is continuously measured in non-invasive condition on the basis of ratio of change of the light absorption of two light beams.
This measurement is performed by fitting a probe to finger or an earlobe. The probe is provided with two light emitting elements (such as light emitting diode) for irradiating red and infrared light different in wavelength and a light receiving element (photodiode or the like) for outputting a photoelectric transfer signal in accordance with a light receiving level of a reflected light or a transmitted light of two light emitting elements, respectively. For example, a wavelength (.lambda.) of 660 nm is used for the red light and a wavelength (.lambda.) of 940 nm is used for the infrared light. The two light beams are irradiated to the living tissue on a time sharing basis.
After that, a degree S of the oxygen saturation is measured from a ratio of a light absorption change which is calculated on the basis of the photoelectric transfer signal relating to each wavelength. The ratio .phi. between the light absorption of the two different wavelengths and the degree S of the oxygen saturation are expressed by the following equation (1) and (2), respectively: EQU .phi.=A1/A2 (1)
A1: light absorption change of wavelength of red light
A2: light absorption change of wavelength of infrared light EQU S=f (.phi.) (2)
f: function of ratio .phi. between light absorption between two wavelengths.
Next, a drive circuit for driving two light-emitting diodes on the time sharing basis is described hereinafter.
FIG. 8 is showing a circuit diagram indicating conventional drive circuit for driving the light emitting element on the pulse oximeter. As shown in FIG. 8, a red and an infrared light emitting diodes are driven by two independent drive circuits, respectively. This drive circuit corresponds to a transistor switching circuit as generally used. A light emitting diode 21a for emitting a red light is connected in a forward direction between the collector of switching transistor 20a and a supply end of the direct current voltage supply VDD. An emitter of a switching transistor 20b is grounded and a base of the switching transistor 20b is connected in series to a resister R22a for inputting a control pulse PR therein.
A light emitting diode 21b for emitting an infrared light is connected in a forward direction between the collector of switching transistor 20b and a supply end of the direct current voltage supply VDD. An emitter of a switching transistor 20b is grounded and a base of the switching transistor 20b is connected in series to a resister R22b for inputting a control pulse PR therein.
Next the operation of the conventional drive circuit is described hereinafter.
FIG. 9 is a timing chart showing a light emission timing of each light emitting diode operated by a control pulse. Control pulses PR and PR shown in FIGS. (a) and (c), respectively, are changed to a high level (H) or a low level (L) for irradiating the light emitting diode on the time basis. Namely, when the control pulse PR is in the high level, the switching transistor 20a turns on (conducting) so that the light emitting diode 21a is irradiated as shown in FIG. 9(b) and when the control pulse PR is in the low level, the switching transistor 20a turns off (non-conducting) so that light emitting diode 21a is in non-light emission condition.
On the other hand, as shown in FIG. 9(d), when the control pulse PIR is in the high level (H), the switching transistor 20b turns on so that the light emitting diode 21b is irradiated. Further, the control pulse PIR is in the low level (L) so that the switching transistor 20b turns off so that the light emitting diode 21b is in non-irradiative condition. In this case, the two light emitting diodes are alternatively irradiated with a light emission ratio being not more than 50%.
In the conventional device, the starting emission voltage for the light emitting diode is about 2 V. However, it is necessary to apply a high voltage (DC power supply VDD) to the switching transistor in the consideration of a variation of the forward direction characteristic of the light emitting diode and a voltage drop occurred by an internal resistance when the switching transistor 20 turns on. Therefore, the conventional device employs a power supply circuit for converting a commercial AC power supply to DC power.
In the conventional drive circuit for driving the light emitting diode in the pulse oximeter, when the high voltage (DC voltage VDD) is applied to the switching transistor in consideration of the variation of the forward direction characteristic of the light emitting diode and the voltage drop occurred by the internal resistance when the switching transistor 20 turns on, a power supply utilization efficiency is insufficient.
For example, when DC voltage 9 V is applied to an anode of the light emitting diode, a cathode voltage thereof is in approximately 7 V when the light emitting diode is irradiated. Assuming that a current flowing to the light emitting diode considers i, a total power consumption becomes 9.times.i (W). 7.times.i (W) is consumed from the total power consumption 9.times.i (W) when the switching transistor turns on. Since a power consumption of the light-emitting diode is 2.times.i (W), the power utilization efficiency remains at approximately 22% only.
As a result, in the conventional pulse oximeter, notwithstanding it is required to decrease power to be uselessly consumed in the conventional device, it is impossible to decrease the total power consumption.
Although attempts have been made to lower the light emission duty ratio so as to decrease the power consumption, the S/N ratio is reduced. Thus, the measurement of the oxygen saturation may not be performed in accuracy. For the reason described above, there is not provided a portable pulse oximeter which is capable of long measurement with a cell.